1. Field of the Invention
This invention relates to an earphone, and in particular to an earphone of the intra-concha type.
2. Description of the Related Art
Intra-concha earphones are small earphones that are placed, in use, in the user's outer ear, adjacent to the entry to the user's ear canal.
It is known that, in order that the earphone should produce sound with a good low frequency response, the earphone casing should be provided with a port for venting pressure generated by the speaker. It is also known that this port may be provided in the inlet through which the cable enters the earphone casing.
However, it is often advantageous for this port to have a known cross-sectional area, and this cannot usually be achieved when the port is provided in the inlet through which the cable enters the earphone casing, because the movement of the cable can alter the effective cross-sectional area.
Noise cancelling systems are well known, in which a microphone is also included in the earphone casing, for detecting ambient noise. One type of noise cancelling system has an adaptive gain in the noise cancelling circuitry. That is, the earphone is provided with an error microphone, positioned so as to detect the level of ambient noise reaching the wearer's ear canal. The gain applied to the noise cancelling signal is then controlled, based on that level of ambient noise. One issue that arises with such earphones in particular is that, when the gain is set to a high level, and the venting port becomes coupled to the microphone for detecting ambient noise (for example by the wearer's finger approaching the earphone), this will be interpreted as a very high level of ambient noise, and the noise cancelling system will generate a very loud sound in an attempt to overcome that ambient noise.
It is therefore advantageous for the venting port to be located well away from the microphone for detecting ambient noise.